1. Field of the Invention
The invention generally relates to a semiconductor device and method of manufacture and, more particularly, to a complementary metal-oxide-semiconductor (CMOS) device that includes shallow trench isolations (STI) overhanging interfaces thus preventing oxidation induced compressive stresses in the device.
2. Background Description
CMOS devices may be fabricated using various process techniques. One method entails fabricating strained Silicon (Si) layers on relaxed Silicon Germanium (SiGe) layers. As the Germanium (Ge) concentration increases, strain in the Si lattice increases. This is significant because such strain affects performance (e.g., electron and hole mobility). While strain may improve electron mobility in n-channel field effect transistors (nFETs), performance improvement (i.e., enhancement of hole mobility) in p-channel field effect transistors (p-FETs) poses a greater challenge. Hole mobility in a pFET initially exhibits a slight degradation at low amount of tensile strain, but increases linearly with higher strain.
Compressive stress applied in the longitudinal direction with respect to the current flow may cause a significant increase in hole mobility, but may also degrade electron mobility. The shallow trench isolation (STI) process commonly used in CMOS fabrications to isolate discrete components to prevent interference is susceptible to volume expansion induced stress caused by oxidation. This stress can substantially affect performance, such as adversely by decreasing nFET electron mobility.
In particular, Si located adjacent to the vertical portion of an STI is susceptible to oxidation induced stress. The Si may become oxidized during gate oxidation or reoxidation of a gate stack. The oxidized portion may exhibit significantly increased thickness due to the use of multiple gate oxidations, which is common in fabricating high performance logic circuits. The increased thickness induces stress in the silicon active area, which can affect performance, such as adversely by decreasing nFET electron mobility.
The effect on performance of such oxidation induced compressive stresses is magnified when the source of the stress is close to a transistor gate. Modern CMOS chips have millions of active devices side by side in a common silicon substrate. As efforts to miniaturize and incorporate more active devices on a single substrate continue, it becomes increasingly likely that such sources of stress will be close enough to appreciably impact performance.
The invention is directed to overcoming one or more of the problems as set forth above.